This invention relates to numerical control equipment for a machine tool with measuring devices such as a tool measuring device, tool presetting device, workpiece measuring device, three-dimensional measuring device and digitizer, and more particularly to numerical control equipment which performs accurate measurement with a measuring head, on the principle of electrical micromeasurement.
Development of numerical control equipment depends greatly on automatic operation of machine tools or other industrial machines. The automatic operation is improved by the automation of a stage changing operation, a setting operation, a workpiece dimension measuring operation, and an error correcting operation. In this connection, a tool measuring operation, a tool presetting operation and a workpiece measuring operation have been automated earlier. The tool measuring operation and the work measuring operation are employed for automation of the workpiece dimension measuring operation and the error correction operation. The tool presetting operation is employed for automation of the setting operation.
The term "tool measuring operation or tool measurement" as used herein is intended to mean the operation in which the preset position of an edge of a tool and the actual position thereof are subjected to comparison measurement. For instance in a machining system, with respect to a preset value for the length of protrusion of a tool with respect to the machine origin of an end mill edge as a reference position, variation including the edge wear caused by the machining operation with the tool and the thermal displacement of the mechanical system is measured at the edge. The difference between the variation thus measured and the preset value is obtained so as to rewrite the tool wear correcting data of the numerical control equipment.
Therefore, if the tool wear correcting data (in the case of the end mill, tool diameter correction in a radial direction X, and tool length correction in a lengthwise direction Z), is used, then the following workpiece can be machined by the end mill to initially set dimensions or configuration with the X-axis and Z-axis correction.
The term "tool presetting operation" as used herein is intended to mean the operation in which a machine reference position is made to coincide with a tool position in a tool chamber or the like, the length of protrusion of the tool from the reference position to the edge is obtained in X-Z coordinates with a tool microscope or the like, and is written as tool preset data for the numerical control equipment, or it is measured with a measuring device for the tool preset on the machine tool so as to be automatically written in memory.
For instance in a machining system, the distance between the edge of a tool connected to a machine to a machine origin is measured in the X-Y-Z coordinates in such a manner that the tool is secured to the spindle head and the edge of the tool is held in contact with the measuring device, so that it is automatically written as tool presetting data for the numerical control equipment. Therefore, even if a machining program is formed with tools different in the X-Y-Z coordinates of the edges regarded as if they were equal in length, correction is automatically effected by using the preset data for every tool selected for a machining operation. Accordingly, even when the tool length (Z) or the tool diameter (X or Y) changes, the workpiece can be finished to desired dimension with the same program.
The term "workpiece measuring operation" as used herein is intended to mean the operation in which the dimensions of a machined workpiece are measured in the X-Y-Z coordinates, and, when necessary, errors in dimension and in configuration are calculated.
In order to eliminate the deviation of the dimensions or configuration of the machined workpiece from those set initially because of the errors caused by the tool wear and/or the thermal displacement of the mechanical system, measurement is carried out for every machining operation or several machining operations to obtain the difference between the measured values and the initially set values, so that for instance the tool wear correcting data and the tool diameter correcting data are updated. Accordingly, before the following workpiece is machined, the correction is carried out so that workpiece is machined exactly to the initially set dimensions or configuration. The measured values and the errors in dimension or configuration may be printed out for preparation of a workpiece inspection result.
The workpiece measuring operation is necessary for machining a workpiece to initially set dimensions or configuration. For this purpose, each machining point of a machined workpiece is measured in the X-Y-Z coordinates on the machining center, the differences between the measured values and initially set values are calculated so that a tool wear correcting value is determined to update the tool wear correcting data. For instance when the diameter of an inner circular hole formed with an end mill is smaller, the tool diameter correcting data inputted is reduced as much as the difference, and when the depth of the circular hole from the end face is smaller, then the tool length correcting data inputted is reduced as much at that difference.
Thus, the workpiece to be worked can be machined under the conditions where the errors in dimension or configuration involved in the preceding machining operation have been eliminated.
The tool measuring operation, the tool presetting operation and the work measuring operation are applicable to CNC lathes, CNC jig grinders, and CNC jig borers in the same manner.
FIG. 3 outlines the arrangement of a conventional numerical control equipment with a measuring device in which an X-axis of coordinate axis of a vertical type machining center is exemplified. Specifically, the equipment will be described with reference to the case where a workpiece measuring operation is given to a block-shaped object 8 on the machining center.
In FIG. 3, reference numeral 1 designates a numerical control equipment body which comprises a control unit 2, a memory unit 3, a position register 4, an input interface 5, and an output interface 6.
Reference numeral 7 designates a table on which the object 8 under measurement is placed. In the case where the object 8 is in the form of a block, the object 8 is fixedly secured to the table 7 with a clamp or the like in such a manner that it is in contact with a table reference block 7b.
Reference numeral 9 designates a threaded bar engaged with a threaded hole formed in a protrusion 7a extended downwardly from the table 7; 10, a drive motor for rotating the threaded bar 9 to move the table 7; 11, a rotary sensor coupled to the drive motor 10, the output of the sensor 11 being supplied to the position register 4; and 12, a linear sensor provided on the table 7. The linear sensor 12 can be used in place of the rotary sensor 11 when necessary.
Further in FIG. 3, reference numeral 13 designates a touch sensor mounted on the head of the machining center and fixed in position in the X-axis direction, the detection output of the touch sensor 13 being supplied through the input interface 5 to the control section 2; 14, a display unit connected to the output interface 6; and 15, a printer connected to the output interface 6.
The operation of the conventional numerical control equipment will be described with reference to the case where workpiece measurement is given to a block-shaped object.
The movement of the object to a theoretical position (or for a mathematical distance between a measuring point 100 and the original of a figure with the table reference block 7b as a reference, 50 mm in the case of FIG. 4) is controlled as follows: The control section 2 calculates the distance of 450 mm which is obtained by subtracting 50 mm from the distance 500 mm between origin 102 and the figure origin 104, and determines an instruction value according to the result of calculation. According to the instruction value thus determined, the motor 10 is driven to move the table 7 to the right in FIG. 4. While the table 7 is being moved, the output of the rotary sensor 11 is fed back to the position register 4. As a result, speed and position correcting instructions are supplied to the drive motor by the control section 2 until the moving speed and position meet the instruction values. That is, the object is moved to the theoretical position while the correction is repeatedly carried out so that the moving speed and position agree with the instruction value.
In the movement of the table 7, the rapid traverse speed (24 m/min in maximum at present) of the table 7 is decreased to an extremely low speed, for instance a measuring speed of 13.336 mm/min when the table reaches the position which is several millimeters (0.227 mm in this case) away from the theoretical position, thus providing a measuring gap, in order to ensure the accuracy of the workpiece measurement.
On the other hand, when the contactor 13a of the touch sensor 13 fixed in place touches the block-shaped object 8 under measurement, the touch sensor 13 outputs a contact pulse signal, which is applied through the input interface 5, as an interrupt signal, to the control unit 2.
The control unit 2 performes a variety of controls with interrupt signal intervals (time sharing system). Therefore, if the interrupt signal of workpiece measurement completion reaches the control unit 2 during an interrupt signal interval, it will not received until the end of the interrupt signal interval (2.2 msec). Therefore, the table 7, on which the block-shaped object 8 is placed, is moved on at a speed of 13.336 mm/min, and the content of the position register 4 is changed continuously. When the end of the interrupt signal interval occurs with the lapse of time, the interrupt signal from the touch sensor 13 is received, so that the counting operation of the position register 4 is suspended, and the movement of the table at the measuring speed is stopped.
The measurement value of the position resister 4 is applied, as a present position, to the control unit 2, so that the mutual relation between the present position and the theoretical position (command position/target position) 450 mm is operated to obtain a workpiece measurement value, which is applied through the output interface to the display unit 14 so as to be displayed, and is printed out with the printer when necessary.
The workpiece measurement value and errors are printed out when it is required to provide an inspection sheet with the errors displayed in the ranges of allowable values. In the case where it is necessary to machine the workpiece again with the errors out of the ranges of allowable values, the control unit 2 calculates a tool correction value according to the workpiece measurement errors so that the tool corrections stored in the memory unit 3 are automatically rewritten, and under this condition the machining of the workpiece is started again because a procedure of allowing the positional correction of the following workpiece with respect to the theoretical position has been accomplished. Thus, the configuration and dimensions of the object 8 can be placed in the ranges of allowable values.
Disadvantages of the above-described conventional equipment will be described by using numerical data. for instance when the interrupt signal interval is 2.2 msec and the measuring speed is 13.336 mm/min (or 0.227 mm/sec), the positional dispersion is 0.5 .mu.m. When the measuring gap is 0.227 mm, then the measuring movement time is one second.
The workpiece measurement of the machining center has been described. However, also in the case of a tool presetting operation or tool edge measuring operation, or in the case of a CNC lathe, CNC jig grinder or CNC jig borer, the quick forwarding speed must be decreased to the extremely low speed for measurement.
The conventional numerical control equipment is designed as described above. Therefore, because of the relation between the interrupt signal interval and the moving speed, the precision cannot be maintained without employing the extremely low speed or decreasing the interrupt signal interval. However, the interrupt signal interval cannot be changed with ease, being substantially determined by the hardware employed. Accordingly, it is necessary to change the moving speed to the extremely low speed in the measurement. This means an increase in the measuring time, especially in a multi-point measurement.
The term "multi-point measurement" as used herein is intended to mean the following measurement: In the workpiece measurement of the object 8 in FIG. 3, the machined thickness 50 mm is measured at only one point; however, measurement of a plane at one point is low in reliability. In order to overcome this difficulty, a workpiece is measured at a number of points arranged in matrix form or at random, so that the average, upper and lower values of the measurement values are obtained, and the data are rewritten in the same manner single point measurement.
In the case of a free curved surface, the workpiece measurement may be achieved with the measuring point intervals being made constant. This measuring method is also called multi-point measurement.